Adjust THE particles’ lifespan

Go to the nparticleshape and under Lifespan, change the Lifespan mode to Random range. Set the Lifespan value to something like 0.2 or 0.3 and the Lifespan Random to a similar value. Feel free to use your own values according to your settings – you want the particles to be generated, then stop, and start dying, but feel free to to tweak the timing.